In the field of electronics, a wide variety of filters are used to eliminate or select specific ranges or bands of frequencies. Low-pass filters are used to pass the spectral components of a signal that are below a cutoff frequency of the low-pass filter while attenuating spectral components that are above the cutoff frequency. Conversely, high-pass filters are used to pass the spectral components of a signal that are above a cutoff frequency while attenuating spectral components that are below the cutoff frequency. Band-pass filters are used to pass the spectral components of a signal falling within a predetermined range of frequencies while attenuating the frequency components above and below that range.
The most elementary low-pass and high-pass filters simply use a resistor connected in series with a capacitor. However, the current trend of implementing electronic devices using digital circuitry makes it less desirable to utilize these analog low-pass and high-pass filters. Therefore, various digital filtering techniques have been devised that can easily be implemented by properly programming a microprocessor that receives samples of the signal to be filtered. A digital low-pass filter can be easily derived by modeling the step response of an analog low-pass filter. Basically, the output of the low-pass filter changes by a magnitude equal to the exponential decay during the time between successive samples from the value of the prior output sample to the current input value. Although the step response of an analog low-pass filter can be easily modeled to provide a digital low-pass filter, it is substantially more difficult and processor intensive to implement a digital high-pass filter. In summary, while it has been relatively simple to implement a digital low-pass filter with modest processing requirements, it has not been heretofore possible to easily implement a digital high-pass filter without requiting substantially more processing power.
Another problem with digital high-pass filters results from baseline shift. In many field, such as medical electronics, a relatively low amplitude AC signal rides on a substantially greater DC voltage. In the event that only the AC is of interest, the signal is normally passed through a high-pass filter which then brings the AC signal down to a zero volt baseline. Thereafter, the AC signal can be greatly amplified. High-pass filters are capable of maintaining the AC signal on the baseline only if the rate at which the baseline shifts is relatively slow. If the baseline shifts at a sufficient rate that the spectral components of the shift are above the cutoff frequency of the high-pass filter, the shift will pass through the high-pass filter. Under these circumstances, the signal of the output of the high-pass filter will either saturate amplifiers downstream from the filter or cause a display of the AC waveform to extend beyond the visible bounds of the display.